After a failed attempt to knit the medium size, I started over and knit the large size with several modifications, based on the advice below.

Advice from HilaryWithOneL: Wherever the pattern says “pick up XX stitches,” it should say “pick up and knit XX stitches.”

Advice from Pingla: To ensure a triangle shawl and avoid an undesired rhombus/diamond shape, follow this pattern of decreases:
MC rows:
RS: 4 decreases
WS: 2 decreases
CC rows:
RS: 4 decreases
WS: 0 decreases

Advice from Hinke: For the top edge, pick up 3 stitches for every 4.

One tip of my own that I would add is not to bother keeping a written count of the repeats for the bottom edging; it’s much quicker to simply count the holes created by the double yarn overs, one hole per repeat.

Other Modifications
I chose not to use a garter stitch border along the edge of the striped section--I kept everything in stockinette--to make picking up stitches easier later. I intended to do the same for the lace section too, but forgot until after I’d knitted it.

Progress
01/17/2014 - Re-knit the top edge and bound off without incident. Only blocking remains.

01/16/2014 - I started over from the beginning some time in December but then stalled due to pre-holiday work chaos, holiday travel, and post-holiday lethargy. I finished everything but the top edge by this date. I was nearly through with the top edge and ready to bind off when I yanked on a stray thread and managed to create this weird, huge extra loop dangling from the side of the work. Attempting to fix this resulted in me beginning to unravel one of the border ends. So, after much wailing and gnashing of teeth, I pulled out the top edge.

12/5/2013 - So, uh, yeah. I should have read everyone’s notes about how, even when knitted with the correct weight yarn and the specified needles, even the large shawl was too small. I put in about two solid hours of knitting away on the stockinette body before it dawned on me that there was just no way this thing was going to wrap around my neck and stay on comfortably--it will either constantly slide off or will have to be tied in a choking knot. So, I’m going to let it stew a few days while I decide how to proceed after the inevitable frogging.

11/27/2013 - Finally started the body of the shawl.

11/4/2013 - Picked up border stitches (perfect count on the first try!) and knit the setup row and first row of the chart.

10/24/2013 - Restarted border on size 5 needle.

10/23/2013 - Bought needle and frogged first attempt.

10/22/2013 - Had a 12-hour day at work and no chance to buy a needle. Was very disgruntled.

10/21/2013 - Made it as far as the first two rows of the chart, and then decided that I didn’t really like the way the yarn draped on the size 6. Elected to go down a needle size, which necessitated buying a new needle.

10/11/2013 - Cast on with a size 6 (4 mm) needle in the car on the way to Raleigh. Realized over the weekend that the pattern is written for a sport weight yarn, but figured since my Holden Shawlette was knit on size 6 then it would be fine.